Cancer can be considered to be a group of diseases that can occur in any tissue, organ, or system of the body. The causes of all cancers are not yet known, nor are there any reported major qualitative metabolic differences between cancer cells and host tissue cells of origin. Accordingly, cancer chemotherapy, unlike the chemotherapy of infectious diseases wherein the disease-causing organism itself offers a distinct metabolic or structural biological target, has far more restrictive fundamental concepts on which to pattern therapeutic treatment.
Most known classes of anticancer drugs exert their action principally because of quantitative differences in metabolic rates of production or levels of certain nucleic acids, enzymes, proteins, hormones, metabolic intermediates, etc., rather than because of qualitative biologic differences between cancer cells and normal cells. Thus, anticancer drugs do not exhibit selective toxicity in the classical sense.
Nucleosides, as a specific group of anticancer or antiviral agents, can be taken up selectively into cells via several mechanisms. Once the corresponding nucleotide is formed intracellularly, the nucleotide is then available for conversion into diphosphates, triphosphates, etc., and thereby can exert its cytotoxic effect via a number of possible mechanisms including effects on DNA polymerase, ribonucleoside diphosphate reductase, incorporation into DNA, and inhibition of DNA and cellular metabolism in general.
A number of anticancer nucleosides or bases have been described in the prior art. For example, cytosine arabinoside (ara-C), 5-fluorouracil, 5-fluorodeoxyuridine, 6-mercaptopurine, and thioguanine are among drugs currently used in the clinical treatment of cancer in human patients. Literally scores of pyrimidines, purines, structurally related heterocyclic bases, nucleosides, etc., have been synthesized and demonstrated to possess high cytotoxic activity in cell culture and, in a number of instances, in tumor-bearing animals. However, unfavorable therapeutic indexes have restricted the clinical use of this class of antimetabolites to the relatively few antineoplastic drugs presently used for the chemotherapy of cancer.
Recently, a number of compounds have been reported wherein the oxygen of the furanose ring of a number of natural and synthetic nucleosides has been replaced by a methylene group. This transformation changes the furanose ring into a cyclopentane ring. The term carbocyclic nucleoside is used to describe these compounds which are structurally analogous to natural and synthetic nucleosides wherein the furanose ring is replaced by a 5-member carbon ring. It is perhaps more accurate to refer to these compounds as carbocyclic nucleoside isosteres because, strictly speaking, they are not nucleosides. Carbocyclic nucleosides, however, is a convenient term because these compounds undoubtedly exert their biological activity by mimicking the parent nucleosides, although their activities may be different for a variety of reasons. Consistent with the presence of the carbocyclic ring, they are not subject to the action of nucleoside phosphorylases and hydrolases that cleave normal nucleosides. Conformationally, however, the expected similarity in bond lengths and bond angles between the tetrahydrofuran and cyclopentane rings allows these analogues to behave as substrates or inhibitors of the enzymes that activate and interconvert nucleosides and nucleotides in living cells. As a result of this likeness, many of these compounds possess an interesting range of biological activities, particularly in the areas of antiviral and anticancer chemotherapy. The majority of carbocyclic nucleosides known to date are of synthetic origin, although two of the most active compounds are natural products: aristeromycin and neplanocin A.
Several carbocyclic nucleosides were conceived and synthesized prior to the isolation of the carbocyclic adenosine prototype aristeromycin from natural sources. Some of the initially synthesized compounds were simple cyclopentyl substituted bases, but others included true isosteres of thymidine and adenosine. The first reported synthesis of carbocyclic thymidine however, was found to be in error, but the correct compound was later prepared. Most current synthetic approaches begin with the construction of the heterocyclic base from a functionalized cyclopentylamine which, with very few exceptions, is obtained as a racemic mixture. Consequently, most of the reported synthetic carbocyclic nucleosides are racemates. Recently, however, an enantioselective synthesis of aristeromycin and neplanocin A was achieved by Ohno et al., as reported in J. Am. Chem. Soc. 105, 4049 (1983). Of the three total syntheses of neplanocin A reported in 1983, two are enantioselective.
The basic method of synthesis of carbocyclic nucleosides has remained substantially unchanged since Shealy's original work, published in J. Am. Chem. Soc, 88, 3884 (1966), and J. Am. Chem. Soc. 91, 3075 (1969). This synthesis involved:
(1) synthesis of the carbocyclic ribofuranosylamine (C--rib--NH.sub.2), and
(2) construction of the purine or pyrimidine ring from this amine by well established procedures in nucleoside chemistry. The other syntheses that followed differed mainly in the novelty and efficiency of producing the desired C--rib--NH.sub.2 with the correct stereochemical disposition of substituents.
Most of the syntheses used a rigid bicyclo[2.2.1]heptene system, which allowed for better control of the stereochemistry of incoming substituents in subsequent reactions. When nonbornadienes were used as starting materials, the extra carbon atom in the molecule was replaced by the required amino function via a Hoffmann rearrangement of a carboxylic acid amide generated after ozonolysis of one of the double bonds. Later, in an effort to overcome the use of the Hoffmann reaction, azabicyclo[2.2.1]heptene systems, which already contain a latent amine functionality, allowed a more efficient generation of C--rib--NH.sub.2. Ohno's use of a chemico enzymatic hydrolysis of a mesodiester allowed synthesis of an enantiomerically pure C--rib--NH.sub.2. Other methods often led to the desired amine only in its racemic form.
Among the various synthetic approaches to purines and pyrimidines, only a few methods have been used in carbocyclic nucleoside chemistry, mainly because of the early commitment to the synthesis via the cyclopentylamine.
To form purines, the time-honored method used has been to convert the carbocyclic amine to the corresponding pyrimidylaminocyclopentane derivative which is then followed by completion of the pyrrole, imidazole, or triazole ring, to give the corresponding purine carbocyclic nucleoside. The reactive 6-chloro substituent allows replacement with ammonia or water to give the adenine and hypoxanthine analogues, respectively. Completion of the bicyclic system varies accordingly; it consists of (1) a spontaneous acid-catalyzed cyclization; (2) formation of the imidazole ring after treatment with an activated one-carbon reagent such as triethylorthoformate; or (3) diazotization of the primary aromatic amine to give the 8-azapurine analogue.
All reported syntheses of carbocyclic pyrimidines have made use of preformed carbocyclic amines as starting materials. The procedures apply the general methodology for the synthesis of uracil and thymine. An acyl isocyanate derivative is reacted with the carbocyclic amine to give an intermediate acryloylurea which is then cyclized in the presence of concentrated ammonia, or with acid catalysts, to give the uracil or thymine analogue. Alternatively, the same result can be obtained by reacting the carbocyclic amine with 3-ethoxy-N,2-bis(ethoxycarbonyl)acrylamide to give the 5-carboethoxyuracil. The 5-substituent was later removed by hydrolysis and decarboxylation. The generated unsubstituted carbocyclic uridine derivatives were amenable to direct halogenation at C-5 and the halogen later displaced by a host of nucleophiles to produce a number of 5-substituted uridine analogues.
Transformation of the uracil ring into cytosine requires conversion of the cyclic amide to the 4-chloropyrimidine, which reacts with ammonia. Alternatively, thiation of the uridine analogue to the corresponding 4-thiouracil derivative, followed by methylation and ammonolysis, produces identical results.
Purine carbocyclic nucleosides include compounds with an intact imidazo[4,5-d]pyrimidine (purine) ring system bearing different 9-cyclopentyl substituents that mimic the several known sugar moieties of the corresponding nucleoside counterparts. Other variations include substitutions at positions 6, 2, and 8.
The first of the ribose isosteres that was synthesized was the saturated carbocyclic analogue of adenosine, C-Ado. C-Ado displayed a wide range of biological activities. It was highly cytotoxic to both H.Ep.-2 and L1210 cells in culture, but it demonstrated poor selectivity towards the tumor cells in view of its inactivity in the in vivo mouse L1210 model system. At subtoxic concentrations, it induced cell proliferation of quiescent normal cells, but in contrast, it inhibited growth in malignant cell lines. The primary toxic effects of C-Ado appear to be mainly derived from the corresponding nucleotide (C-AMP) generated in cells containing adenosine kinase. Like adenosine, C-Ado is also deaminated by adenosine deaminase, but its affinity for the enzyme is a hundredfold lower.
All other 6-substituted C-Ado analogous reported have also been found to be ineffective against L1210 leukemia in mice, despite the fact that some of them were found to be cytotoxic to H.Ep.-2 cells in vitro.
The saturated carbocyclic analogue of 3-deazaadenosine (3-deazaaristeromycin) was first reported in 1982 by Montgomery et al in J. Med. Chem. 25, 626 (1982). This compound was found to be a very potent and specific inhibitor of the enzyme which hydrolyzes S-adenosyl-L-homocysteine (AdoHcy). Besides demonstrating good antiviral activity against herpes simplex and vaccinia viruses, it was devoid of some of the undesirable side effects typical of other antiviral agents operating by the same mechanism. The antiviral activity observed for these compounds appears to result from the inhibition of methylation of the 5' cap of viral m-RNA caused by the increase accumulation of AdoHcy inside the cell. Antiviral activity of this nature is discussed by De Clerq et al in Biochem. Biophys. Res. Commun. 129, 306 (1985). Inhibition of this critical methylation reaction hinders the translation of viral m-RNA into viral proteins. A common characteristic shared by 3-deazaadenosine and 3-deazaaristeromycin is resistance towards phosphorylation and deamination, which suggests that the carbocyclic structure plays a significant role in conferring the aforementioned selectivity to 3-deazaaristeromycin.
Recently, a different class of carbocyclic nucleosides has become interesting after the isolation and total synthesis of the fermentation antibiotic neplanocin A. Neplanocin A is also a potent inhibitor of AdoHyc hydrolase, but since it is readily phosphorylated, it has a multiplicity of side effects including cytotoxicity. The important structural feature of neplanocin A is the unsaturation present in its cyclopentenyl ring, which gives the molecule unique pharmacologic properties when compared with its saturated counterpart, aristeriomycin.